In a mobile communication system including plural cells, when a user equipment terminal (UE: user equipment) moves from one cell to another, the user equipment terminal switches to the other cell to continue communications. The switching to the other cell is referred to as handover. Typically, when the user equipment terminal moves to a neighbor cell and signal strength in the neighbor cell is higher than signal strength in the serving cell (cell in which the user equipment terminal originally performs communications), the user equipment terminal performs handover to the neighbor cell.
Specifically, the user equipment terminal performs handover according to the procedure shown in FIG. 1.
First, the user equipment terminal measures signal power in the neighbor cell (S1). Then, the user equipment terminal determines whether the signal power in the neighbor cell satisfies the following condition.signal power in the neighbor cell+offset>signal power in the serving cell
When this condition is satisfied, the user equipment terminal reports an event (Event A1) to the network (base station) (S2). It should be noted that the offset is provided so as to avoid frequent handover to the neighbor cell from the serving cell at the cell boundary. The offset may be a positive value or a negative value. When the network receives the event (Event A1), the network determines that the user equipment terminal should perform handover to the cell for which the event is reported, and then performs handover procedures (S3). While the event is defined as Event A1, the event may be defined as any other event such as Event A3.
Quality in handover has a significant influence on communication quality in the mobile communication system. For example, failure in handover is not preferable because it may cause a disconnection during communications or a significant decrease in the transmission rate from the user's point of view.
When the user equipment terminals moves fast such as on a bullet train or a highway, the handover procedures need to be quickly performed in order to maintain communications. When the duration needed for the handover procedures is longer than the length of time during which the user equipment terminal moves to the adjacent cell, communications are disconnected, because the user equipment terminal enters into the coverage area of the neighbor cell (moves out of the coverage area of the serving cell) before switching to the neighbor cell.
Handover in an LTE (Long Term Evolution) system, which is a succeeding system of the W-CDMA (Wideband Code Division Multiple Access) system or the HSDPA (High Speed Downlink Packet Access) system, is described below in detail. As shown in FIG. 2A, a variable system bandwidth ranging from six resource blocks (RBs) to one hundred resource blocks can be used in the LTE system, depending on capabilities of the base station and the user equipment terminal. For example, the system bandwidth of six resource blocks, fifty resource blocks, or one hundred resource blocks may be used. The resource block is a single transmission unit in the frequency domain. The frequency bandwidth of one resource block is equal to 180 kHz (see TR 36.804, V0.8.0, Section 5.1, TR 36.803, V0.7.0, Section 5.1, and TS 36.211 V8.0.0). In the LTE system, the system bandwidth is also referred to as a channel bandwidth.
In the LTE system, RSRP (Reference Signal Received Power) is used for a handover criterion (see TS 36.214, V8.0.0, Section 5.1.1). Other than RSRP, RS-SIR (Reference Signal Signal-to-Interference Ratio), E-UTRA Carrier RSSI (Received Signal Strength Indicator), RSRQ (Reference Signal Received Quality), or the like may be used for the handover criterion.
FIG. 2B shows a conceptual diagram of measurement of RSRP in the case where the system bandwidth is equal to 20 MHz (one hundred resource blocks). The user equipment terminal may measure RSRP within the bandwidth of six resource blocks, within the bandwidth of fifty resource blocks, or within the bandwidth of one hundred resource blocks.